Recent data from our laboratory and others strongly suggest that oxytocin (OTN), a hypothalamic peptide, is also present in the corpus luteum (CL). The observation that this hormone is present in a reproductive organ in high concentrations is particularly significant because in addition to its established role in parturition and lactation OTN can modulate progesterone and testosterone synthesis, induce the synthesis and/or release of PGF 2Alpha by the endometrium and affect the length of the estrous cycle. Although the presence of OTN in the CL does not in itself indicate synthesis by this tissue the reported high level raises this possibility. We porpose to determine whether OTN is a secretory product of the CL, study its secretion pattern, determine its subcellular localization and identify its function. We intend to achieve these objectives by: 1) isolating sufficient material for amino acid composition and sequence in order to determine the authenticity of this putative OTN. Amino acid analysis will be performed by reverse phase HPLC of fluorescent derivatives and the sequencing will be done on a 860C Beckman automatic amino acid sequencer; 2) using an established CL explant culture system study the incorporation of radiolabeled amino acids into luteal peptides and subsequently characterize these radiolabeled peptides by using co-elution on reverse phase HPLC, SDS-gel electrophoresis and immunoprecipitation; 3) studying the granule population of luteal cells using peroxidase-antiperoxidase immunohistochemistry at the light microscopy level and protein A gold at the electron microscopy level; and 4) developing and validating a sensitive radioimmunoassay in order to study the secretion pattern of luteal OTN throughout the estrous cycly and pregnancy. As this work progresses other peptides isolated from the CL will be biochemically characterized using the above approach.